In the normal pluralization rules of Sindarin, the form eryd would be the proper plural, because an o in the final syllable of a singular noun became y in the plural, such as emyn the plural of amon "hill" and gelydh the plural of golodh "Noldo". To explain the presence of ered in the Lord of the Rings, Tolkien considered modifying the internal history the Sindarin pluralization rules, so that the y-plurals of the First Age had in most cases changed to e by the Third Age, except in limited cases such as before nasals like n and m (to explain emyn, which also appeared in the Lord of the Rings).[5][note 1]

Notes

↑ Tolkien even noted: "Use Eryd in Silmarillion". Perhaps Christopher used Ered in The Silmarillion to be consistent with The Lord of the Rings (J.R.R. Tolkien, "Words, Phrases and Passages in Various Tongues in The Lord of the Rings", in Parma Eldalamberon XVII (edited by Christopher Gilson), p. 33.